Changing to a bigger tire size on the Yamaha MT-125 is not recommended as it can affect the bike's handling, stability, and overall performance. The recommended tire sizes for the Yamaha MT-125 are 100/80-17 for the front and either 130/70-17 or 140/70-17 for the rear, depending on the model year. Using larger tires than these specifications can lead to issues with clearance, suspension alignment, and speedometer accuracy. It is best to stick with the manufacturer-recommended tire sizes to ensure optimal performance and safety.